Originally Posted by wakespeak
We're stuck with 14.7:1 fuel/air mixture. Big engines suck in more air which in turn needs more fuel. So displacement works against you. Therefore to get the best fuel economy you need smaller displacement. Turbo/blower engine would be one method, AFM the other. Either would need to be supplemented with reduced weight and better aerodynamics.
In either case, in part throttle it would run on reduced displacement, getting your 30 mpg. The turbo/blower effectively increases displacement by forcing in more air, so your 3.5 liter can be a 6 liter with boost. AFM of course just reduces/add cylinders.
I think GM should raise the price of Camaros (or at least the V8 models) to limit their numbers and play the CAFE averaging game that way. This would keep the cars at higher margins/quality/performance. Create a different fuel economy oriented car for the buyers that are about fuel economy eg Cruze SS, 130R etc.
BMW eventually went the turbo route. I hate AFM myself, so I would rather have a turbo v-6 or pay a gas guzzler tax when buying the v-8.
Adding a turbo doesn't always increase fuel economy. The new GM Turbo V6 has near identical power and torque as the LS3 and gets the same gas mileage. The friction in an engine is a big determinant on fuel economy. A smaller engine might have to work harder, aka higher RPMs or throttle, than a larger engine to maintain a certain speed. In that case the increase in fuel economy might not be as noticeable because both engines could very well be using the same mass of air. The larger engine would require less throttle, and while it will "suck" in a larger volume of air for every stroke that volume of air will be at a lower pressure due to a greater pressure drop at the throttle and could be very similar in mass to that of a smaller motor using more throttle or spinning more RPMs. Also, when designing a car around turbos, the compression ratio must usually be lowered for reliability and longevity. This decreases the efficiency of the engine while it is not under load.
When the car goes into AFM mode, it may only be putting fuel into 4 cylinders, but it still has the friction of 8. Therefore, the other 4 have to work a bit harder, aka more throttle, which equals more air entering the 4 working cylinders. The only gain is you don't have as many combustion losses out of 4 cylinders vs 8 cylinders. That is why the gain in MPG seems minimal.
BMW went the turbo route mostly because it is a European based company. Most countries in Europe tax a car based on engine displacement. It's not a very fair system because nothing is an apples to apples comparison.